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AFTER induction of anesthesia, tracheal intubation often causes lung resistance increases (bronchoconstriction). 1–5 This constrictive response presumably is initiated by activation of abundant laryngeal and tracheal receptors with reflex constriction of the peripheral airways. 2

Forskolin, a direct activator of adenylate cyclase, 6 is known to cause relaxation of the airway smooth muscles similar to other agents that increase intracellular cAMP. 7–12 The results form experimental study by Hiramatsu et al.13 using guinea pig tracheal smooth muscle and porcine tracheal myocytes suggested that this relaxation is mediated at least in part by opening the large-conductance calcium Ca2+-activated potassium channels; however, the potential usefulness of forskolin in treating bronchospasm 14 is limited by its poor water solubility.

We recently reported that intravenous colforsin daropate, a novel and potent water-soluble forskolin derivative, prevents bronchoconstriction induced by intravenous administration of thiamylal and fentanyl in combination under tracheal intubation and found that colforsin daropate is a potent bronchodilator. 15 We hypothesized that prophylactic bronchodilator treatment with intravenous colforsin daropate, with a bronchodilating effect in animals 16–18 and humans, 15 before tracheal intubation would result in decreased airway resistance and increased lung compliance after placement of the endotracheal tube when compared with placebo medication. Thus, we also measured hemodynamics and catecholamines because these variables can be affected by intravenous colforsin daropate, which has positive inotropic and vasodilatory actions. 15

Patients and Methods

Patients

After obtaining approval from our institutional review board (Chiba Hokusoh Hospital, Nippon Medical School, Chiba, Japan) and written informed consent from the study patients, 46 adult patients with American Society of Anesthesiologists physical status classification of I or II who were scheduled to undergo minor elective surgery were enrolled in the study. Patients who had a clinical or radiologic abnormality of the ventilatory system, had a suspected (history of atopy) or overt (history of wheezing) bronchial hypersensitivity, or were receiving treatment with a β-blocker were excluded from the study. A random number computer-generated program was used to assign study patients randomly to one of two groups: (1) a placebo (control) group (n = 23) or (2) a colforsin daropate group (n = 23).

We drew arterial blood to measure plasma epinephrine and norepinephrine at baseline (just before the study began) and 30 min after the study began (just before anesthesia induction). The study was completed before the elective surgery was initiated. We analyzed differences between smokers and nonsmokers after completion of this study.

Statistical Analysis

For statistical analyses, we used chi-square analysis to compare differences in sex between the control group and the colforsin group. Unpaired t
tests were applied to compare differences in age, weight, and height between the control group and the colforsin group. Unpaired t
tests were also applied to compare the differences in cigarettes per day between the control group and the colforsin group. Intragroup comparisons of systolic and diastolic arterial pressure, heart rate, Rawm, Rawe, and Cdynwere performed by two-way analysis of variance with repeated measures and paired t
tests with the Bonferroni correction. Between-group comparisons were made at each time point by unpaired t
test. Paired and unpaired t
tests were used to compare differences in plasma epinephrine and norepinephrine. The mean ± SD is given for each value. A P
value less than 0.05 was considered statistically significant.

Results

Preoperative pulmonary function test results did not differ between the control group and the colforsin group, and they did not differ between smokers and nonsmokers (data not shown).

Table 1shows the demographic data for the two groups. There were no statistical differences between the two groups. Demographic data for smokers and nonsmokers are shown in table 2.

Arterial blood pressure did not change in either group after treatment (fig. 1, top
). Although heart rate increased after colforsin daropate infusion, it did not change after normal saline infusion (fig. 1, bottom
). After anesthesia induction, heart rate decreased compared to baseline values in the control group; however, it remained increased after treatment in the colforsin group (fig. 1, bottom
).

Patients receiving colforsin daropate had significantly lower Rawmand Raweat 4, 8, 12, and 16 min after intubation (fig. 2, top
and center
), and they had significantly higher Cdynat 4, 8, 12, and 16 min after intubation (fig. 2, bottom
). In the control group, Rawmdecreased at 8, 12, and 16 min after intubation when compared with Rawm4 min after intubation (fig. 2, top
), and Rawedecreased at 8 and 12 min after intubation when compared with Raweat 4 min after intubation (fig. 2, center
). However, Rawmand Raweremained unchanged in the colforsin group (fig. 2, top
and center
). In the colforsin group, Cdyndecreased at 16 min after intubation when compared to Cdynat 4 min after intubation (fig. 2, bottom
).

Smokers in the control group had a Rawmvalue higher than that of nonsmokers at 4 min after intubation (fig. 3, top
). After 4 min, Rawmwas similar in these two group (fig. 3, top
). However, smokers and nonsmokers in the colforsin group both had a similar Rawmat all time points (fig. 3, bottom
).

Fig. 3. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for mean airway resistance. †P
< 0.05 vs.
nonsmokers. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for mean airway resistance.

Smokers and nonsmokers in both the control and colforsin groups had a similar Raweafter intubation (fig. 4).

Fig. 4. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for expiratory airway resistance. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for expiratory airway resistance.

Fig. 4. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for expiratory airway resistance. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for expiratory airway resistance.

Smokers in the control group had a lower Cdynthan that of nonsmokers 4 min after intubation (fig. 5, top
). After 4 min, both had a similar Cdyn(fig. 5, top
). In the colforsin group, however, Cdynat all time points was similar for smokers and nonsmokers (fig. 5, bottom
).

Fig. 5. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for dynamic compliance. †P
< 0.05 vs.
nonsmokers. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for dynamic compliance.

Fig. 5. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for dynamic compliance. †P
< 0.05 vs.
nonsmokers. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for dynamic compliance.

Both groups had comparable baseline concentrations of plasma epinephrine, and the plasma epinephrine concentration remained unchanged after injection in both groups (table 3). Both groups had comparable baseline concentrations of plasma norepinephrine, and even though plasma norepinephrine concentrations increased after medication in the colforsin group, the concentrations did not change in the control group even after injection (table 4).

These observations suggest that prophylactic bronchodilator treatment with intravenous colforsin daropate before tracheal intubation resulted in lower airway resistance and greater dynamic lung compliance after placement of the endotracheal tube when compared with placebo medication. Moreover, Rawmwas higher for smokers than for nonsmokers in the control group, and it was similar for smokers and nonsmokers in the colforsin group. Cdynwas lower for smokers than for nonsmokers in the control group, and it was similar for both smokers and nonsmokers in the colforsin group.

Thus, colforsin daropate was shown to be an effective bronchodilator in humans. Forskolin is a direct activator of adenylate cyclase 6 and is known to cause a relaxation of airway smooth muscle similar to other agents that increase intracellular cyclic adenosine monophosphate (cAMP). 7–12,27,28 Although it is well acknowledged that an increase in the level of cAMP is associated with the relaxation of tracheal smooth muscle, the precise molecular events underlying cAMP-mediated relaxation are not known. 17,29 The involvement of many different mechanisms has been suggested. For example, an increase in cAMP may reduce the affinity of myosin light chain kinase for the Ca2+-calmodulin complex through the phosphorylation of myosin light chain kinase by cAMP-activated protein kinase. This would result in a decrease in the Ca2+sensitivity of the contractile elements. 30 Alternatively, cAMP may reduce intracellular Ca2+by enhancing the Ca2+extrusion to the extracellular space via
an activation of sarcolemmal Ca2+–adenosine triphosphatase and/or an increase in sodium–Ca2+exchange secondary to the activation of sodium–potassium pump. 29 Ca2+sequestration into intracellular storage sites may also be facilitated by cAMP, leading to a decrease in intracellular Ca2+. 29 Observations using patch clamp techniques have shown that large-conductance Ca2+-activated potassium channels are distributed abundantly in the surface of airway smooth muscle cells 31,32 and that these channels are stimulated via
cAMP-dependent phosphorylation as well as by a cAMP-independent, membrane-delimited signal transduction process. 32–34 Activation of Ca2+-activated potassium channels should cause the membrane hyperpolarization of smooth muscle cells; this hyperpolarization is expected to inhibit the Ca2+influx through voltage-dependent Ca2+channels. 35 Satake et al.17 reported that with use of guinea-pig isolated tracheas, the bronchorelaxant action of colforsin daropate may result, at least in part, from activation of Ca2+-activated potassium channels, which may cause a hyperpolarization of smooth muscle cell membranes and a secondary decrease in Ca2+influx through voltage-dependent Ca2+channels, leading to a decrease in intracellular Ca2+.

Kil et al.1 showed that there was no difference in lung resistance between smokers and nonsmokers in their placebo group; however, in our study, 4 min after intubation, smokers had higher levels of Rawm(fig. 3, top
) and lower levels of Cdyn(fig. 5, top
) than did nonsmokers in the control group. Although we cannot explain why our results differed from those of Kil et al.
, 1 we speculate that differences in the study groups may be responsible. The smoking patients in the study of Kil et al.1 had lower ratio of 1-s forced expiratory volume to vital capacity (percent) and forced expiratory flow after 25–75% of expelled vital capacity (percent predicted) values than nonsmoking patients, and the patient group had a moderate degree of obstructive lung disease, especially among smokers. Moreover, their patients were approximately 20 yr older than ours.

Kil et al.1 also showed that after treatment with ipratropium bromide, an anticholinergic bronchodilator, and albuterol, a β2-adrenergic agonist, postintubation lung resistance was lower for treated nonsmokers than for treated smokers. Our results showed that after treatment by intravenous colforsin daropate, Rawm, Rawe, and Cdynwere similar for smokers and nonsmokers after tracheal intubation (figs. 3 through 5, bottom
). These results suggest that intravenous colforsin daropate treatment before intubation is beneficial and advantageous for smokers. Kil et al.1 commented as follows:“The lesser response to bronchodilators in smokers are often said to have reactive airways. However, this result may reflect a higher fixed resistance in smokers that in nonsmokers. The airway response to tracheal intubation may be a normal reflex response that may even be blunted in smokers by the presence of chronic irritation and inflammation. However, this remains speculation.” The differences in study groups noted above may be a reason for this difference.

Although they administered inhaled ipratropium bromide and albuterol to their patients, we gave colforsin daropate intravenously. Intravenous administration for 30 min may not be the best method; inhalational may be an alternative or better method. We believe that a suitable inhalational method needs to be developed. In this study, we chose 0.75 μg · kg−1· min−1as the dosage of colforsin daropate because the appropriate clinical dose range of intravenous colforsin daropate for acute heart failure is considered to be from 0.25 to 0.75 μg · kg−1· min−1. 36 However, we could not determine the optimal dose of colforsin daropate in the current study, and further investigation is needed. Colforsin daropate induced tachycardia (fig. 1, bottom
), which was consistent with previous reports. 37,38 This drug has positive inotropic and vasodilatory actions, makes the cardiac index increase, the pulmonary capillary wedge pressure decrease, the stroke volume increase, and the systemic vascular resistance decrease. 37 Thus, the drug improves hemodynamics in patients with acute congestive heart failure. 37 The effects of this drug are mediated by an increase in intracellular cAMP concentration caused by the stimulatory action of this drug on adenylate cyclase and not through β-adrenoreceptors. Results from several animal experiments suggest that this agent may be effective in patients with severe heart failure who fail to respond to β-stimulants or phosphodiesterase inhibitors. 37

We recently showed the thiamylal–fentanyl combination induces bronchoconstriction. 15 Cigarini et al.39 reported that 5 mg/kg thiopental followed by a 15-mg · kg−1· h−1continuous infusion with a 5-μg/kg fentanyl bolus injection induces bronchoconstriction. Cigarini et al.39 showed that, under thiopental anesthesia, fentanyl induced a small but highly significant increase in maximum tracheal pressure and respiratory resistance associated with a decrease in respiratory compliance. We suggested that the release of histamine is probably not involved in the bronchoconstriction induced by thiamylal and fentanyl. 15

Plasma epinephrine concentrations remained unchanged after intravenous colforsin daropate (table 3), and plasma norepinephrine concentrations increased significantly after treatment in the colforsin group (table 4), but we consider the change to be very small and not clinically relevant. These results show that catecholamines are not involved with the lower levels of Rawmand Raweand higher levels of Cdynin the colforsin group after intubation compared with those in the control group. Hirota et al.18 noted recently in dogs that catecholamine release may not be responsible for colforsin daropate–induced relaxation.

The results of our study suggest that colforsin daropate is a bronchodilator in humans, and we speculate that colforsin daropate may be useful as a bronchodilator in the treatment of bronchial asthma, similar to other agents that increase intracellular cAMP. Adrenergic down-regulation can occur rapidly in many tissues. Therefore, β2-agonists might have a rapidly decreasing effect in time, which is a potential problem for the treatment of bronchial asthma. 40 Colforsin daropate may even be effective in patients with bronchial asthma who fail to respond to β-stimulants because the action of this drug is not mediated through β-adrenoreceptors. However, further clinical investigation is required to confirm such speculation.

In conclusion, prophylactic treatment with colforsin daropate produced lower airway resistance and higher dynamic lung compliance after tracheal intubation when compared with placebo medication. Moreover, our results suggest that treatment with colforsin daropate before anesthesia induction and tracheal intubation is beneficial and advantageous for middle-aged and smokers without chronic obstructive pulmonary disease.

Fig. 4. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for expiratory airway resistance. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for expiratory airway resistance.

Fig. 4. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for expiratory airway resistance. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for expiratory airway resistance.

Fig. 5. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for dynamic compliance. †P
< 0.05 vs.
nonsmokers. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for dynamic compliance.

Fig. 5. (A
) Comparison between smokers (▪) and nonsmokers (□) in the control group for dynamic compliance. †P
< 0.05 vs.
nonsmokers. (B
) Comparison between smokers (▪) and nonsmokers (□) in the colforsin group for dynamic compliance.